1. Field of the Invention
This invention relates to an optical unit for use in an image forming apparatus, more specifically, an optical unit for guiding a light beam from a laser element to a recording medium via a group of lenses and an optical deflector.
2. Description of the Related Art
An image forming apparatus represented by a laser beam printer apparatus includes an optical unit for supplying a laser beam having a desired beam spot size to a recording medium.
In the optical unit, a light beam emitted from a laser element serving as a light source is converted through a first optical system to a parallel beam or a convergent beam having a desired beam spot size. The light beam with the desired beam spot size is deflected by an optical deflector. The beam spot size of the deflected light beam is adjusted by a second optical system to a beam spot size suitable for use in a printer apparatus, and the light beam is focused on the recording medium.
The first optical system has, in combination, an aspherical glass lens, plastic lens, etc., for changing the beam spot size of the light beam traveling from the laser to the deflector to a desired size. The aspherical glass lens and plastic lens are integrally assembled via a support member such as a lens barrel. The deflector has a reflecting mirror(s) or polygonal mirror which has a flat surface or curved surface and is rotated by a motor, etc. The second optical system includes an f.theta. lens for making the rotation angle of the mirror proportional to the distance from the optical axis center in the main scan direction of the light beam focused on the recording medium or photoconductor, with respect to the light beam traveling from the optical deflector to the photoconductor.
In the optical unit, in order to keep an optimum beam spot size of the light beam traveling from the laser to the deflector, at least one of the lenses of the first optical system is designed to be movable along the optical axis. In this case, the movable lens is moved along the optical axis, thereby maintaining an optimum beam spot size.
There is a case where at least one lens (a glass lens in general) of the first optical system and the laser element are arranged integrally via a lens barrel. In this case, the beam spot size of the light beam is not easily affected by a variation in temperature or humidity.
On the other hand, the lens barrel is generally secured, via a leaf spring and screw, to a holding member for securing the base or lens barrel of the optical unit.
However, since at least one lens (or lens barrel) of the first optical system is movable, the position of the lens (in particular the lens barrel) may be displaced when slight vibration or shock acts on either the lens barrel or optical unit or when the printer apparatus is moved roughly. This poses a serious problem in maintaining the optimal beam spot size.
Even where the lens barrel (lens) is fixed, for example, to the leaf spring by an adhesive, if the urging force of the leaf spring upon the lens barrel is strong, the lens barrel may be displaced by the urging force of the leaf spring. In addition, when the amount of the adhesive is small, the lens barrel may easily be affected by vibration or shock.
The displacement of the fixed lens (in particular the lens barrel) causes, for example, a decrease in resolution of output characters or figures, deformation of characters or distortion of figures, when the optical unit is built in the printer apparatus. In addition, when the position of the lens barrel is displaced, the optical unit, in usual cases, must be re-assembled by using many checking devices. As a result, the printer apparatus cannot be used for a long time. In the case where the optical unit is replaced, instead of re-assembling the optical unit, the cost of outputs (hard copies) obtained by the printer apparatus increases remarkably since the optical unit is very expensive.